I presume that you mean the computer shuts down when the battery is at 40%.
if you haven't, go to power options in the control panel. on the left side of the
page there will be "change when the computer sleeps". on that page there
will be "change advanced power settings". when the applet opens, scroll
down to battery and expand it. you will find a whole array of settings that only
apply when you are running on battery. (depending on the motherboard,
your settings may not take without hacking the registry.)
also in the advanced applet there is a "restore defaults" option.
good luck.